The present disclosure relates to equipment used during subterranean formation operations and, more particularly, to permanent electrode sensors that enable galvanic measurements in downhole applications.
Hydrocarbons are typically produced from wellbores drilled from the Earth's surface through a variety of producing and non-producing subterranean zones. The wellbore may be drilled substantially vertically or may be drilled as a lateral well that has some amount of horizontal displacement from the surface entry point. A variety of servicing operations may be performed in the wellbore, such as mechanical intervention services and formation property evaluations, after it has been drilled by lowering different kinds of downhole tools into the wellbore. For example, measuring instruments are commonly lowered into the wellbore to obtain various downhole conditions, such as depth-dependent formation pressure and temperature, formation porosity, fluid viscosity, and density. Various sampling and/or logging devices are also commonly lowered into the wellbore to either analyze fluid samples in-situ at various target zones of the subterranean formation or extract formation fluids for surface laboratory analysis.
The logging devices may include galvanic monitoring systems that typically use monopole or dipole electric sources. A measured signal results from a relatively high excitation level that is delivered from the surface. Such systems include separated excitation electrodes that force current to flow in a surrounding formation. Conduction currents flow within the formation producing a net voltage drop and induced magnetic fields. The galvanic monitoring systems include electromagnetic (EM) receivers that can sense a small signal representing a desired property of the formation. In electric receiver systems, the net voltage drop is sensed. In magnetic receiver systems, the induced magnetic fields are sensed. The receivers are positioned between the excitation electrodes along the tools length.
Since both wires of the excitation source and measurement sensors reside within a borehole, direct cross coupling formed between the excitation source and the measurement sensors limit the system's sensitivity to a desired formation measurement. This direct cross coupling is referred to as crosstalk coupling. The intended electric or magnetic field measurements are susceptible to errors resulting from the crosstalk coupling, such as time varying magnetic fields that are produced by the wires that carry the current to the excitation electrodes.